Process for the separation of aromatic hydrocarbons from a mixed hydrocarbon feedstock

ABSTRACT

A continuous process for separating aromatic hydrocarbons including benzene and toluene from a mixed hydrocarbon feedstock containing same utilizing two extraction zones and a distillation train wherein light aliphatic impurities are removed from the benzene in an azeotropic distillation zone under specified conditions.

United States Patent Kosseim et al.

[ Jan. 29, 1974 PROCESS FOR THE SEPARATION OF AROMATIC HYDROCARBONS FROM A MIXED HYDROCARBON FEEDSTOCK Inventors: Alexander J. Kosseim, Yorktown Hgts; George S. Somekh, New Rochelle, both of NY.

Assignee: Union Carbide Corporation, New

Y York, NY.

Filed: Oct. 6, 1972 Appl. N0.: 295,521

U.S. Cl 260/674 SE, 203/27, 203/81, 208/314, 208/315, 208/321 Int. Cl C07c 7/10 Field of Search 260/674 SE; 208/314, 315

[56] References Cited UNITED STATES PATENTS 3,492,365 l/l970 Anderson et al. 208/314 3,725,254 4/1973 Wang 208/314 Primary Examiner--Herbert Levine Attorney, Agent, or FirmPaul A. Rose et al.

[5 7 ABSTRACT A continuous process for separating aromatic hydrocarbons including benzene and toluene from a miXed hydrocarbon feedstock containing same utilizing two extraction zones and a distillation train wherein light aliphatic impurities are removed from the benzene in an azeotropic distillation zone under specified conditions.

8 Claims, 1 Drawing Figure PROCESS FOR THE SEPARATION OF AROMATIC l-IYDROCARBONS FROM A MIXED HYDROCARBON FEEDSTOCK FIELD OF THE INVENTION This invention relates to an improvement in a process for the separation of aromatic hydrocarbons from a mixed hydrocarbon feedstock and, more particularly, to the recovery of high purity aromatic hydrocarbons therefrom.

DESCRIPTION OF THE PRIOR ART U.S. Pat. No. 3,492,365 issued on Jan. 27, 1970 to John R. Anderson and George S. Somekh, which is incorporated by reference herein, describes a process for the separation of aromatic hydrocarbons utilizing two extraction steps to provide an extract comprising the aromatic hydrocarbons from the original mixed feed dissolved in a so-called secondary solvent, the extraction medium in the second step. This solution or secondary extract is then subjected to distillation to separate the aromatic hydrocarbons from the solvent, the aromatics being subjected to further distillation to recover specific aromatics and the extraction solvents being recycled.

With the advent of the benzene-toluene-C aromatics fraction (known and hereinafter referred to as BTX) as the principal raw material in the manufacture of petrochemicals, outstripping ethylene in this regard, and the increased demand for aromatics as a component in gasoline to increase its octane rating and thus reduce or eliminate the need for lead, which has been under fire as a pollutant, aromatics processes availed of in the past have come under close scrutiny with an eye toward improving process economics. At the same time, industrial demands for benzene of extremely high purity have increased.

SUMMARY OF THE INVENTION An object of this invention, therefore, is to provide an improvement in the process of US. Pat. No.

3,492,365 for the separation of aromatic hydrocarbons from a mixed hydrocarbon feedstock whereby high purity benzene can be recovered while effectively using process heat, apparatus, and components to optimize process economics.

Other objects and advantages will become apparent hereinafter.

According to the present invention, aromatic hydrocarbons are effectively separated from a mixed hydrocarbon feedstock containing aliphatic and aromatic hydrocarbons, said aliphatic hydrocarbons being comprised, at least in part, of light aliphatic hydrocarbons having 5 to 7 carbon atoms in their molecular structure, and specific aromatic components recovered, particularly high purity benzene, by a continuous process comprising the following steps:

a. introducing the feedstock into a primary extraction zone at about the midpoint thereof and contacting said feedstock with a primary solvent and a secondary solvent in said primary extraction zone at a temperature in the range of about C. to about 150C. and a pressure in the range of about atmospheric pressure to about 200 psia wherein the primary solvent is a watersoluble organic solvent, which has a higher boiling point than and is non-azeotropic with the feedstock,

and the secondary solvent is selected from the group consisting of paraffinic and naphthenic hydrocarbons and mixtures thereof, said hydrocarbons having higher boiling points than and being non-azeotropic with the feedstock, and wherein the primary solvent is maintained in sufiicient amount to extract essentially all of the aromatichydrocarbons from the feedstock and the secondary solvent is maintained in sufiicient amount to act as a reflux for the feedstock;

b. withdrawing from the primary extraction zone primary extract comprising aromatic hydrocarbons, primary solvent, and about 0.01 to about 5 percent by weight based on the weight of the feedstock of the light aliphatic hydrocarbons defined above, and primary raffinate comprising aliphatic hydrocarbons and secondary solvent;

c. contacting said primary extract with secondary solvent in a secondary extraction zone wherein the temperature and pressure are in the same range as in step (a) and the secondary solvent is maintained in sufficient amount to extract essentially all of the aromatic I hydrocarbons from the primary extract;

(1. withdrawing from the secondary extraction zone primary solvent and secondary extract comprising aromatic hydrocarbons, secondary solvent, and the light aliphatic hydrocarbons defined above;

e. subjecting the primary raffinate to distillation in a raffinate distillation zone whereby the aliphatic hydrocarbons are separated from the secondary solvent;

f. withdrawing the secondary solvent from the raffinate distillation zone and recycling said secondary solvent to the secondary extraction zone;

g. recycling the primary solvent withdrawn from the secondary extraction zone to the primary extraction zone;

h. recycling a sufficient amount of secondary extract to the primary extraction zone to provide the secondary solvent required in step (a);

i. subjecting the balance of the secondary extract to distillation in a secondary extract distillation zone whereby a mixture comprising aromatic hydrocarbons and the light aliphatic hydrocarbons defined above is separated from the secondary solvent;

j. subjecting the mixture of step (i) to distillation in a benzene distillation zone to separate a mixture comprising benzene and the light aliphatic hydrocarbons defined above from the aromatic hydrocarbons having a higher boiling point than benzene;

k. subjecting the mixture defined and separated in step (j) to azeotropic distillation in an azeotropic distillation zone wherein i. the temperature at the bottom of the zone is at about the boiling point of benzene;

ii. the temperature at the top of the zone is in the range of about 30C. to about 170C;

iii. the pressure at the bottom of the zone is about 5 psia to about 140 psia; and iv. a portion of the overhead is recycled to the zone as reflux at a reflux ratio of about 10/1 to about 300/ l to separate an azeotropic mixture of benzene and the light aliphatic hydrocarbons defined above from high purity benzene wherein the high purity benzene is at least percent by weight of the benzene in the feedstock;

1. subjecting the aromatic hydrocarbons having a higher boiling point than benzene and separated in step (j) to distillation in a toluene distillation zone to separate toluene from the aromatic hydrocarbons having a higher boiling point than toluene wherein i. the temperature at the top of the toluene distillation zone is about 5C. to about 60C. higher than the temperature in the bottom of the azeotropic distillation zone;

ii. the pressure at the top of the toluene distillation zone is about 2 psia to about 100 psia; and

iii. a portion of the overhead is recycled to the toluene distillation zone as reflux;

m. supplying a major proportion of the heat for the azeotropic distillation zone by heat exchange between a portion of the benzene leaving the azeotropic distillation zone as bottoms and the overhead from the toluene distillation zone, and recycling said portion to the azeotropic distillation zone in a vapor state; and

n. recovering high purity benzene from the azeotropic distillation zone, and toluene and aromatic hydrocarbons boiling at a higher temperature than toluene as overhead and bottoms, respectively, from the toluene distillation zone.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE is a schematic flow diagram of an illustrative embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As noted above, there is an industrial need for BTX, which is available in high proportion, e.g., greater than 30 percent by weight in a wide variety of hydrocarbon feedstocks such as reformed gasolines; coke oven light oils; cracked gasolines; and dripolenes, which, after hydrogenation, can contain as much as 70 to 98 percent BTX. These feedstocks also contain both saturated aliphatic and cycloaliphatic hydrocarbons, which can be present in amounts of 5 to 85 percent by weight of the feedstock and can be predominantly C to C aliphatics or even higher where prefractionation has been accomplished. Since the individual hydrocarbon compounds which make up these feedstocks are well known, they will not be discussed extensively; however, it can be pointed out that the major components of the feedstocks are hydrocarbons with boiling points ranging from 60C. to 200C including straight-chain and branched-chain paraffins and naphthenes, such as nheptane, isooctane, and methyl cyclohexane, and aromatics such as BTX.

The BTX fraction can include benzene, toluene, the C aromatics ortho-xylene, meta-xylene, para-xylene, and ethyl benzene, and C aromatics, which, if present at all, appear in the smallest proportion in relation to the other components.

Other feedstocks containing the aforementioned components, which would be suitable for use in the system described hereinafter are petroleum distillates, naphthas, and catalytic reformates and pretreated and prefractionated fractions of the various feedstocks mentioned above.

We are primarily concerned here with those aliphatics in the feedstock that become a part of the primary extract and pass into the secondary extraction zone. These aliphatics can be defined as saturated light aliphatic hydrocarbons having 5 to 7 carbon atoms in their molecular structure. The amount, which gets into the primary extract as impurities in the process described here, is about 0.01 to about 5 percent by weight based on the weight of the feedstock initially introduced into the system. It will be understood by those skilled in the art that a small amount of C and C aliphatics also get into the primary extract and the distillation train, but since these latter impurities follow the toluene and C aromatics rather than the benzene, they do not interfere with the high purity of the benzene recovered here, and, for that reason, are not included in the definition of light aliphatics set forth above. These defined C to C lights are essentially all removed in subject process and are, optionally, recycled to the primary extraction zone.

The primary solvent is defined above as a watersoluble organic solvent, which has a higher boiling point than and is non-azeotropic with the feedstock. It is also polar and essentially water-free. Examples of solvents which may be used in the process of this invention as primary solvents are dipropylene glycol, tripropylene glycol, dibutylene glycol, tributylene glycol, ethylene glycol, diethylene glycol, propylene glycol, butylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, sulfolane, N-methyl pyrrolidone, triethylene glycol, tetraethylene glycol, ethylene glycol diethyl ether, propylene glycol monoethyl ether, pentaethylene glycol, hexaethylene glycol, dimethylsulfoxide, and mixtures thereof. The preferred group of solvents is polyalkylene glycols and the preferred solvents are tetraethylene glycol and sulfolane. It should be noted than the feedstock and primary solvent must be selected so that the definition of the primary solvent, set forth above, is satisfied, i.e., with respect to boiling point and azeotropic characteristics.

The selectivity of many of the primary solvents can be enhanced by the addition to the solvent of so-called anti-solvents such as ethylene glycol, diethylene glycol, propylene glycol, and butylene glycol and mixtures thereof. These glycols were mentioned above as examples of primary solvents and can be used therefor under certain conditions. The choice of solvents and antisolvents with a view towards optimizing the process is generally dependent upon the composition of the feedstock introduced into the primary extractor, the degree of separation required, and other operating variables which are involved.

The secondary solvent is defined above as a solvent selected from the group consisting of paraffinic and naphthenic hydrocarbons and mixtures thereof, said hydrocarbons having higher boiling points than and being non-azeotropic with the feedstock. The secondary solvents contain a maximum of about one percent by weight aromatic hydrocarbons and preferably no more than about 0.5 percent by weight aromatic hydrocarbons. The paraffinic hydrocarbons can be either straight or branched chain. As for the primary solvent, both the feedstock and secondary solvent must be selected so that the definition of the secondary solvent is satisfied. Examples of secondary solvents which may be used alone or together are n-decane, n-dodecane, 2- methyl decane, 2,2-dimethyl decane, n-hexyl cyclohexane, 2-methyl hexyl cyclohexane, tridecane, ntetradecane, n-pentadecane, n-hexadecane, n-

heptadecane, n-octadecane and n-nonadecane. Generally, a mixture in the form of a fraction wherein the components boil within a particular range is used. The primary solvent is substantially immiscible with the non-aromatic hydrocarbons present in the feedstock and the secondary solvent is substantially immiscible with the primary solvent.

The apparatus used in the process both for extraction and distillation is conventional, e.g., an extraction column of the multistage reciprocating type containing a plurality of perforated plates centrally mounted on a vertical shaft driven by a motor in an oscillatory manner can be used as well as columns containing pumps with settling zones, sieve trays with upcomers, or even a hollow tube while the distillation can be conducted in a packed or bubble plate fractionating column. Countercurrent flows are utilized in both extraction and distillation columns.

REFERRING TO THE DRAWING The feedstock is introduced as a liquid phase through line 101 at about the mid-point or middle tray of primary extractor 103; primary solvent, essentially free of water (trace amounts may be present), is introduced at the top tray of primary extractor 103; and secondary solvent is introduced below the bottom tray of primary extractor 103. Both the primary and secondary solvents come initially from outside sources (not shown) until steady state conditions are achieved and then only make-up solvents are supplied from the outside sources.

The function of the secondary solvent in the primary extractor is to act as a reflux, i.e., to purify the aromatics present in the feedstock.

As stated above, the primary solvent is introduced into and maintained in the primary extractor in sufficient amount to extract essentially all of the aromatic hydrocarbons from the feedstock and the secondary solvent is introduced into and maintained therein in sufficient amount to act as a reflux for the feedstock. Generally, to accomplish the extraction the ratio of primary solvent to feedstock in the primary extractor is in the range of about 1 to about parts by weight of primary solvent to one part by weight of feedstock and preferably in the range of about 2 parts to about 9 parts of primary solvent per part of feedstock. ln final analysis, however, the ratio is selected by the technician based on experience with the particular feedstock and depends in part upon whether high recovery or high purity is being emphasized. The same is true of the secondary solvent, the suggested ratio of primary solvent to secondary solvent in the primary extractor being in the range of about 1 part to about 30 parts by weight of primary solvent to one part by weight of secondary solvent and, preferably, about 5 parts to about parts by weight of primary solvent per part of secondary solvent.

It should be noted that any ratios or amounts given herein for various components mean the ratio or amount of the particular component present in one cycle of the described process.

The temperature in the primary extractor is in the range of about 50C. to about 150C. and is preferably in the range of about 80C. to about 110C.

The pressure in the primary extractor is in the range of about atmospheric pressure to about 200 pounds per square inch absolute (psia) and preferably about 30 psia to about psia. As is well known in the art, however, one selected pressure is not maintained throughout the extraction zone, but, rather, a high pressure within the stated range is present at the bottom of the zone and a low pressure again within the stated range is present at the top of the zone with an intermediate pressure in the middle of the zone. The pressures in the zone depend on the design of the equipment and the temperature, both of which are adjusted to maintain the pressure within the stated range, which is important if the preferred mode of taking a liquid overhead in the extraction zones is to be achieved.

The raffinate is such a liquid taken overhead. It includes the feed aliphatics (except for the C to C impurities discussed above), essentially all of the reflux secondary solvent, some entrained primary solvent, and may contain some C aromatics in very small proportion. The raffmate leaves primary extractor 103 via overhead line 111 and passes to a heat exchanger (not shown) where it is heated to about its boiling point and may be partially vaporized if desired thus reducing the heating requirements in the rafflnate stripper. In this heated state, the raffinate then passes into raffinate stripper 113 via line 111 where the bulk of the aliphatics along with some C aromatics, if any, are removed as a vapor overhead through line 131. A portion of the raffinate is returned to the top tray of rafflnate stripper 113 through line 132 where it is condensed (condenser not shown) as a reflux, which aids in purifying the raffinate by knocking down the high boilers at a reflux ratio of about 0. III to 5/1. The secondary solvent (including extrained primary solvent and which may include other high boiling aliphatics from the feed) is removed as bottoms and passes through line 133 where part is diverted through a reboiler (not shown) and returns to raffinate stripper 113 below the bottom tray as a vapor to provide most of the heating requirement. The balance of the secondary solvent proceeds via line 133 to line and into the secondary extractor to a point below the bottom tray.

The primary extract, which includes the feed aromatics, the bulk of the primary solvent, a small amount of secondary solvent entrained in the primary solvent, and about 0.01 to about 5 percent by weight based on the weight of the feedstock of the light aliphatic hydrocarbons heretofore defined along with a small amount of C and C aliphatics as noted passes as bottoms from primary extractor 103 through line 107 to the top of secondary extractor 109.

lnitially, secondary solvent is fed into secondary extractor 109 below its bottom tray from a reservoir (not shown) to provide secondary solvent in sufficient amount to extract essentially all of the aromatic hydrocarbons from the primary extract. Generally, the ratio of secondary solvent used in the secondary extractor to the initial feedstock is in the range of about 0.1 part to about 6 parts by weight of secondary solvent per part by weight of feedstock and, preferably, about 0.2 part to about 5 parts by weight of secondary solvent for each part by weight of feedstock. These ratios are especially relevant for the solvent of preference or like solvents; however, although such ratios may be used for other solvents, it is suggested that the technician select ratios based on experience with particular feedstocks as mentioned for the other ratios above. The same reservoir is used to provide make-up secondary solvent to maintain the described ratio in the secondary extractor. After the cycle has started up, however, the bulk of the secondary solvent is that which is recycled from the distillation zones.

The temperature and pressure in the secondary extractor is kept within the same range as that described above for the primary extractor.

The primary solvent essentially free of aromatics, with the small amount of dissolved and entrained secondary solvent becomes the bottoms of the secondary extractor and returns to the top of the primary extractor via line 105. The primary solvent thus avoids high distillation temperatures which lead to its degradation.

The feed aromatics extracted from the primary extract; the bulk of the secondary solvent, i.e., at least about 90 percent by weight thereof; a small amount of primary solvent entrained in the secondary solvent; and about 0.01 to percent by weight light aliphatic hydrocarbons based on the weight of the feedback make up the secondary extract, which passes as a liquid overhead via line 117. The aliphatic hydrocarbons included in the secondary extract are mainly the defined light aliphatics having 5 to 7 carbon atoms in their molecular structure and boiling in the range of about 50C. to about 100C. Insofar as the aromatics to be recovered in the within process, these light aliphatics are considered impurities, although, in accordance with the principle of utilizing process components, these light aliphatics after separation may be recycled to the primary extractor.

A portion of the secondary extract is returned to primary extractor 103 to provide the reflux requirement, i.e., the amount of secondary extract returned to the primary extractor is sufficient to provide the secondary solvent required to act as a reflux for the feedstock as discussed above. The balance of the secondary extract passes into line 127 and through a heat exchanger (not shown) where it is heated to its boiling point and may be partially vaporized and then passes into the middle of secondary extract stripper 119. Vapor is carried overhead in stripper 119 and a portion passes through line 123 where it is condensed (condenser not shown).

Both secondary extract stripper 1 l9 and primary raffinate stripper 113 can be operated at a temperature in the range of about 60C. to about 250C. and at subatmospheric, atmospheric, or superatmospheric pressure, preferably, however, at subatmospheric pressures in the range of about 3 to about 14 psia.

The condensate is returned through line 123 to the top tray of stripper 119 as a reflux. The ratio of the amount of product returned as reflux to the amount of product recovered from the distillation column is known as the reflux ratio of the distillation column. It is advantageous to have a low reflux ratio since less heat is then needed to vaporize the product in the column. It is found that the recycling of secondary extract from the secondary extractor to the primary extractor has the ultimate effect of lowering the reflux ratio in stripper 119. It is further found that running this embodiment without recycling the secondary extract increases the reflux ratio by as much as l00 percent (on a weight basis). Finally the high aromatics content of the extract in the distillation zone (which is a result of recycling) reduces the secondary solvent per unit of aromatics and, therefore, makes possible a reduction of distillation column height and diameter. The reflux ratio in stripper 119, generally is in the range of 0.3/1 to 1.2/1 in the instant process, but varies according to the amount of aromatics in the feed to column 119, e.g., the lower the aromatics content of the feed the higher the ratio.

The secondary solvent, which still includes some dissolved and entrained primary solvent, passes as bottoms from stripper 119 via line where a portion of it is directed into a reboiler (not shown) and returns as a vapor to a point below the bottom tray of stripper 119 to provide most of its heating requirement while the balance of the secondary solvent passes through a heat exchanger (not shown) where it is cooled to the extraction temperature and continues through line 115 to secondary extractor 109. As noted above, make-up secondary solvent is introduced at line 115, if needed, to maintain the proper ratio.

The balance of the overhead from stripper 119 passes through line 121 as a vapor and then passes into the middle of benzene stripper 139. The overhead from stripper 119 is comprised of benzene, higher boiling aromatics, including toluene, xylenes, and possibly C aromatics, and the light aliphatic hydrocarbons (described heretofore) based on the weight of the feedstock.

The temperature at the top of stripper 139 is at about the boiling point of benzene and the temperature at the bottom is, generally, the boiling point of the mixture of aromatics boiling at a higher temperature than benzene.

Vapor passes overhead from benzene stripper 139 through line 141 where it is condensed (condenser not shown) and part of the condensate is returned through line 140 to stripper 139 as reflux. The reflux ratio in stripper 139 is usually about 0.5/1 to about 10/1; however, it depends, in the main, on the number of trays; the ratio of overhead to bottoms product; and the degree of separation desired.

The balance of the overhead from stripper 139, which is in a condensed state, continues through line 141 and may be passed through a heat exchanger (not shown) to be heated to about the boiling point of benzene. It may be partially vaporized if desired. The stream then enters purification (or azeotropic distillation) column 143 at about its midpoint through line 141, and undergoes azeotropic distillation. The overhead stream is removed as vapor through line 144, a portion of which passes through line 153, is condensed therein (condenser not shown), and returned to purification column 143 as reflux. The reflux ratio is set forth below. The balance of the vapor continues along line 144, is also condensed (condenser not shown), and

may be, and is preferably, returned to join line 101,'

which goes into primary extractor 103 at about its midpoint, the benzene therein being thus returned to the system.

High purity benzene, which can have a freezing point of about 5.45C. or even higher is removed as bottoms through line 145, a portion being directed through line 154, into heat exchanger 15] and then heat exchanger 152, if necessary, (the heat being supplied by an outside source, which is not shown) and then returning as vapor to purification column 143 to provide most or all of its heating requirement. It will be noted that the bulk of the heat is supplied by vapor coming from toluene stripper 146 so that the heating cost of running the purification column is minimal. As noted, additional heat, if necessary, is supplied by an outside source.

The amount of heat contributed by the toluene vapor to the purification column varies with the composition of the feedstock particularly the quantities and ratios of C and heavier aromatic components in the feedstock.

Returning to benzene stripper 139, the bottoms comprising aromatics having a higher boiling point than benzene; particularly toluene, xylenes, and C aromatics pass through line 142 to toluene stripper 146. Toluene vapor passes through line 155 into heat exchanger 151 where it provides heat for the benzene coming from purification column 143. It then passes through condenser 156. A portion of the then liquid toluene is returned to stripper 146 along line 155 as reflux, the reflux ratio in toluene stripper 146 usually being about 0.5/1 to about 8/1. This reflux ratio is dependent, for the most part, on the same characteristics of the system as the reflux ratio for the benzene stripper. The balance of the toluene passes through line 147 to a storage tank (not shown). High quality toluene is recovered here.

In order to produce benzene of high purity and to take full advantage of process heat, the following operating conditions are observed in purification column 143 and toluene stripper 146. The temperature at the bottom of purification column 143 is maintained at about the boiling point of benzene (80.1C. at atmospheric pressure i 10C., adjustments to be made where pressure varies from atmospheric) while the temperature at the top of column 143 is maintained in the range of about 30C. to about 170C., the particular temperature selected being dependent on the particular pressure used. The pressure at the bottom of column 143 is maintained in the range of about 5 psia (pounds per square inch absolute) to about 140 psia and is preferably maintained in the range of about 7 psia to about 30 psia. The pressure maintained at the top of the column evolves from a combination of bottom pressure and temperature in the column and lies in the range of about 2 psia to about 135 psia.

The reflux ratio used in purification column 143 can vary from about 10/1 to about 300/1 and is usually in the range of about 50/1 to about 150/1. This reflux ratio depends on the kinds of non-aromatic impurities in the benzene stream flowing into column 143 through line 141, the relative proportion of each with respect to one another and the benzene, on the number of trays in the column, and on the quality requirement for the final pure benzene. The heating requirements of column 143 depend, in turn, on the reflux ratio used in the column and on the amount of the overhead stream passing through line 144.

The amount of heat contributed by the toluene vapors to purification column 143 varies with the composition of the feedstock, particularly the quantities and ratios and C s and heavier aromatic components in the feed. Alternatively, when the feed contains sufficient amounts of C and heavier aromatic compounds, a xylene stripper may be introduced into the system and the overhead vapors therefrom used as a source of heat for purification column 143. This source can serve alone or together with vapors from the toluene stripper or other strippers. Where the feedstock does not contain benzene, but rather toluene and higher aromatics the same system can be used with column 139 as a toluene stripper and column 146 as a xylene stripper.

The temperature at the top of the toluene stripper is maintained at a higher level than that of the bottom of the purification column. For efficient utilization of the heat energy of the toluene vapors in line 155, a temperature differential in the range of about 5C. to about 60C. and preferably between about 10C. and about 50C. is maintained. The pressure at the top of toluene stripper 146 can range between about 2 psia and about psia depending on the particular head temperature employed in the column.

The following example illustrates the invention: Parts, percentages, and ratios are by weight unless otherwise specified.

EXAMPLE Results are those obtained after steady state conditions are achieved.

The extractor stages are primarily theoretical stages having essentially 100 percent tray efficiency.

The feedstock has the following composition:

Component Percent benzene 13.2 toluene 15.9 C and C aromatics 24.1 cyclohexane 0.2 methylcyclopentane 1.2 hexanes 16.5 heptanes 10.0 octanes 9.0

nonanes total 1 00.0

The drums containing primary solvent and secondary solvent are maintained at about C. so that the solvents will enter the extraction columns at about 100C. The primary solvent, feedstock, and secondary solvent are fed into primary extractor 103 at the first stage, between the eleventh and twelfth stages, and below the twenty-second stage, respectively.

Primary extractor 103 is a 22 stage mixer-settler thermostated at 100C.

After steady state conditions are reached the source of the secondary solvent in. the primary extractor becomes the secondary extract.

The primary extract then passes along line 107 to the first stage at the top of secondary extractor 109 where it enters at a temperature of about 100C. Secondary extractor 109 is identical to primary extractor 103 in all respects except that it has 18 stages. The secondary solvent also enters secondary extractor 109 at about 100C. along line at a point below the 18th stage. Again, secondary solvent initially comes from an outside source until steady state conditions are reached and then only make-up solvents are introduced from outside sources.

The extractor operation is carried out under the following conditions:

temperature in both extractors 100C.

primary solvent/feedstock ratio 9.0/1 reflux/feedstock ratio in primary extractor 1.0/1 secondary solvent/feedstock ratio in secondary extractor 4.35/1 pressure in both extractors 50 psia The primary extract contains about 0.1 per cent by weight based on the weight of the feedstock of C to C light aliphatic hydrocarbons.

All the strippers used here have the equivalent of about 20 theoretical stages except for columns 139 and 146, which have the equivalent of 30 theoretical stages.

The overhead from primary extractor 103 passes through line 111 to primary raffinate stripper 113, which is operated at a reflux ratio of about 0.5/1, a bottom pressure of 8 psia and a temperature of about 210C. A raffinate product essentially free of aromatics is obtained overhead through line 131.

The secondary extract from the top of secondary extractor 109 passes through line 1 l7 and is split into two portions. One portion returns through line 117 to the bottom of primary extractor 103 as reflux and the other portion passes through line 127 into secondary extract stripper 119 operated at a reflux ratio of unity, a bottom pressure of 8 psia, and a temperature of about 210C. Aromatic hydrocarbons plus impurities pass overhead through line 121, and secondary solvent passes as bottoms through line 135. The aromatic hydrocarbons stream is then fractionated in benzene stripper 139 operated at about atmospheric pressure, and a temperature of about 90C., at the top and with a reflux ratio of about 3/1. Benzene plus impurities (e.g., cyclohexane, methylcyclopentane, n-hexane and isohexanes) are taken overhead through line 141. The balance of the aromatic hydrocarbons pass as bottoms through line 142 into toluene stripper 146.

The benzene stream in line 141 is introduced into about the middle of purification column 143, which has approximately 20 theoretical stages. Column 143 is operated at a reflux ratio of about 100/ 1 and with a temperature and pressure at the bottom of approximately 77C. and 13 psia, respectively. The temperature at the top of purification column 143 is about 65C. and the pressure at the top is about 9 psia. (The boiling point of benzene at atmospheric pressure is 80.lC.). Benzene having a purity of 99.97 percent is obtained as bottom product through line 145, and about 99.5 percent by weight is recovered from the feedstock.

The overhead from column 143 taken through line 144 is a mixture of benzene azeotropes formed with the light aliphatic hydrocarbon impurities. Stream 144 is condensed (condenser not shown) and split into two parts. One portion is used as reflux through line 153 and the remainder is returned to primary extractor 103 at the feed point. The amount of this recycled stream is about 0.008 pound for each pound of feedstock introduced into the primary extractor. The azeotropic mixture has the following approximate composition:

Component Percent benzene 90.2

-Continued Component Percent hexanes 5. l methylcyclopentane 4.1 cyclohexane 0.6

total 1 00.0

The aromatic hydrocarbons boiling at a higher temperature than benzene pass through line 142 into toluene stripper 146, which is operated under a pressure of about 19 psia at the top of the column and at a reflux ratio of about 4/1. The temperature at the top of the toluene stripper is about C. The toluene obtained overhead through line 147 is of very high purity, about 99.9 percent, an additional advantage of this system.

The recovery of aromatic hydrocarbons from the feedstock is greater than 99.5 percent for benzene and toluene and about 95 percent for the xylenes.

The heat of condensation of overhead vapor from toluene stripper 146 is about 210 BTU per pound of feedstock while the heat requirement of purification column 143 is about 150 BTU per pound of feedstock (the feedstock referred to is that introduced into the system at the primary extractor).

US. Pat. No. 3,492,365, referred to above, is now reissue U.S. Pat. No. Re. 27,492 issued on Sept. 26, 1972.

What is claimed is:

1. A continuous process for separating aromatic hydrocarbons from mixed hydrocarbon feedstock containing aliphatic and aromatic hydrocarbons, including benzene and toluene, said aliphatic hydrocarbons being comprised, at least in part, of light aliphatic hydrocarbons having 5 to 7 carbon atoms in their molecular structure, comprising the following steps:

a. introducing the feedstock into a primary extraction zone at about the midpoint thereof and contacting said feedstock with a primary solvent and a secondary solvent in said primary extraction zone at a temperature in the range of about 50C. to about 150C. and a pressure in the range of about atmospheric pressure to about 200 psia wherein the primary solvent is a water-soluble organic solvent, which has a higher boiling point than and is non-azeotropic with the feedstock, and the secondary solvent is selected from the group consisting of paraffinic and naphthenic hydrocarbons and mixtures thereof, said hydrocarbons having higher boiling points than and being nonazeotropic with the feedstock, and wherein the primary solvent is maintained in sufficient amount to extract essentially'all of the aromatic hydrocarbons from the feedstock and the secondary solvent is maintained in sufficient amount to act as a reflux for the feedstock;

b. withdrawing from the primary extraction zone primary extract comprising aromatic hydrocarbons, primary solvent, and about 0.01 to about 5 percent by weight based on the weight of the feedstock of the light aliphatic hydrocarbons defined above, and primary raffinate comprising aliphatic hydrocarbons and secondary solvent;

c. contacting said primary extract with secondary solvent in a secondary extraction zone wherein the temperature and pressure are in the same range as in step (a) and the secondary solvent is maintained in sufficient amount to extract essentially all of the aromatic hydrocarbons from the primary extract; d. withdrawing from the secondary extraction zone primary solvent and secondary extract comprising aromatic hydrocarbons, secondary solvent, and the light aliphatic hydrocarbons defined above; e. subjecting the primary raffinate to distillation in a raffinate distillation zone whereby the aliphatic hydrocarbons are separated from the secondary solvent; f. withdrawing the secondary solvent from the raffinate distillation zone and recycling said secondary solvent to the secondary extraction zone; g. recycling the primary solvent withdrawn from the secondary extraction zone to the primary extraction zone; h. recycling a sufficient amount of secondary extract to the primary extraction zone to provide the secondary solvent required in step (a); subjecting the balance of the secondary extract to distillation in a secondary extract distillation zone whereby a mixture comprising aromatic hydrocarbons and the light aliphatic hydrocarbons defined above is separated from the secondary solvent; j. subjecting the mixture of step (i) to distillation in a benzene distillation zone to separate a mixture comprising benzene and the light aliphatic hydrocarbons defined above from the aromatic hydrocarbons having a higher boiling point than benzene; k. subjecting the mixture defined and separated in step (j) to azeotropic distillation in an azeotropic distillation zone wherein i. the temperature at the bottom of the zone is at about the boiling point of benzene;

ii. the temperature at the top of the zone is in the range of about 30C. to about 170C;

iii. the pressure at the bottom of the zone is about psia to about 140 psia; and

iv. a portion of the overhead is recycled to the zone as reflux ratio of about /1 to about 300/1;

to separate an azeotropic mixture of benzene and the light aliphatic hydrocarbons defined above from high purity benzene wherein the high purity benzene is at least 95 percent by weight of the benzene in the feedstock;

l. subjecting the aromatic hydrocarbons having a higher boiling point than benzene and separated in step (j) to distillation in a toluene distillation zone to separate toluene from the aromatic hydrocarbons having a higher boiling point than toluene wherein i. the temperature at the top of the toluene distillation zone is about 5C. to about 60C. higher than the temperature in the bottom of the azeotropic distillation zone;

ii. the pressure at the top of the toluene distillation zone is about 2 psia to about psia; and

iii. a portion of the overhead is recycled to the toluene distillation zone as reflux;

m. supplying a major proportion of the heat for the azeotropic distillation zone by heat exchange between a portion of the benzene leaving the azeotropic distillation zone as bottoms and the overhead from the toluene distillation column, and recycling said portion to the azeotropic distillation zone in a vapor state; and

n. recovering high purity benzene from the azeotropic distillation zone, and toluene and aromatic hydrocarbons boiling at a higher temperature than toluene as overhead and bottoms, respectively, from the toluene distillation zone.

2. The process defined in claim 1 wherein:

i. the reflux ratio in the azeotropic distillation column is about 50/1 to about /1; and

ii. the temperature at the top of the toluene distillation zone is about 10C. to about 50C. higher than the temperature in the bottom of the azeotropic distillation zone.

3. The process defined in claim 1 wherein a portion of the azeotropic mixture of step (k) is recycled to about the mid-point of the primary extraction zone.

4. The process defined in claim 2 wherein a portion of the azeotropic mixture of step (k) is recycled to about the mid-point of the primary extraction zone.

5. The process defined in claim 1 wherein the primary solvent is tetraethylene glycol.

6. The process defined in claim 4 wherein the primary solvent is tetraethylene glycol.

7. The process defined in claim 1 wherein the primary solvent is sulfolane.

8. The process defined in claim 4 wherein the primary solvent is sulfolane. 

2. The process defined in claim 1 wherein: i. the reflux ratio in the azeotropic distillation column is about 50/1 to about 150/1; and ii. the temperature at the top of the toluene distillation zone is about 10*C. to about 50*C. higher than the temperature in the bottom of the azeotropic distillation zone.
 3. The process defined in claim 1 wherein a portion of the azeotropic mixture of step (k) is recycled to about the mid-point of the primary extraction zone.
 4. The process defined in claim 2 wherein a portion of the azeotropic mixture of step (k) is recycled to about the mid-point of the primary extraction zone.
 5. The process defined in claim 1 wherein the primary solvent is tetraethylene glycol.
 6. The process defined in claim 4 wherein the primary solvent is tetraethylene glycol.
 7. The process defined in claim 1 wherein the primary solvent is sulfolane.
 8. The process defined in claim 4 wherein the primary solvent is sulfolane. 